The invention relates to an electric discharge tube, in particular a vacuum tube, or to a discharge lamp, in particular a low-pressure gas-discharge lamp, comprising at least one scandate dispenser cathode which is composed of a cathode body and a coating having an emissive surface, said cathode body including a matrix of at least one refractory metal and/or a refractory alloy and a barium compound which is in contact with the matrix material to supply barium to the emissive surface by means of a chemical reaction with said matrix material. The invention further relates to such a scandate dispenser cathode.
Electron tubes, in particular vacuum tubes, are used predominantly as display tubes in television receivers, as monitor tubes, X-ray tubes, high-frequency tubes and microwave tubes for various applications in any field of machine and installation construction, in medical technology, in diagnostic and measuring devices in workshops as well as in electronic games.
Television and monitor tubes must meet ever higher requirements as regards brightness, resolution, constant picture quality and long-term operating performance. To obtain a higher brightness and a better resolution of the electron beam, higher electron-emission current densities in the tubes are necessary, which can only be achieved by means of improved electron sources, i.e. cathodes. In the middle of the 1980s the requirements could be met by standard oxide cathodes having a long-term emission-current density of 2 A/mm2, whereas currently 10 A/mm2 are required, while much higher emission-current densities are required for the novel high-performance tubes.
Virtually the same applies to the emission-current density and long-term stability of X-ray tubes, high-frequency tubes and microwave tubes.
The emission-current density at a cathode is governed, in accordance with the Richardson equation
I0=ART2exp(xe2x88x92xcfx86/kT)
by the work function at the cathode surface xcfx86 and by the operating temperature T.
At a constant operating temperature T, a cathode can produce a higher
At a constant operating temperature T, a cathode can produce a higher emission-current density as its work function xcfx86 is lower. Alternatively, at a constant current density, a cathode can be operated at lower temperatures as its work function xcfx86 is lower. A lower operating temperature has a positive effect on the service life of the cathode and the discharge tube.
At present, the cathodes having the highest electron emission are scandate dispenser cathodes. The two most important types of scandate dispenser cathodes are the xe2x80x9cmixed matrix scandate cathodexe2x80x99 and the xe2x80x9ctop layer scandate cathodexe2x80x9d. The xe2x80x9cmixed matrix scandate cathodexe2x80x9d is composed of a porous cathode body which is made from tungsten and scandium oxide and which is impregnated with 4 BaO.CaO.Al2O3. xe2x80x9cTop layer scandate cathodesxe2x80x9d are composed of a porous tungsten body which is impregnated with 4 BaO.CaO.Al2O3 and which is covered with a thin coating of tungsten and scandium oxide or SC2W3O12.
During operation of the cathode, a surface complex is formed as a result of a chemical reaction between tungsten, scandium oxide and barium-calcium-aluminate, which surface complex brings about and maintains the high electron emission. As this surface complex is destroyed by the ion bombardment in the tube, it must be recovered constantly. However, scandium oxide is not very mobile, so that the segregation of scandium to form the surface complex is disturbed and the cathode emission during operation of the discharge tube or discharge lamp decreases rapidly. To overcome this drawback, it was proposed in EP 0 317 002 to use scandium-containing metal compounds or alloys which are a compound of scandium and one or more of the metals rhenium, ruthenium, hafnium, nickel, cobalt, palladium, zirconium or tungsten to bring about scandium segregation in the surface of the cathode. The long-term behavior of the cathodes in accordance with EP 0317002 is improved, but the reproducibility of the results is insufficient.
Further, EP 0549034 discloses a cathode comprising a matrix body which is impregnated with an alkaline-earth compound and on the surface of which a coating is provided which contains a refractory metal such as, in particular, tungsten and scandium. A high emission at a low operating temperature as well as a quick recovery after ion bombardment and a long service life are obtained in that the coating comprises at least two layers of a different composition, one layer of a pure metal being applied to the impregnated matrix body, said layer containing scandium as well as a metal having a high melting point such as, in particular, tungsten and/or rhenium, and in that a top coating of a metal having a high melting point such as, in particular, tungsten is applied. These cathodes are preferably manufactured by means of a method in which first pure-metal layers of scandium and/or rhenium are manufactured by means of, in particular, a plasma-activated CVD process, preferably by means of a plasma produced by direct-current glow discharge, whereafter the final layer, being a metallic tungsten layer, is provided by means of a CVD process. However, cathodes of this type have a low emission-current density.
Therefore, it is an object of the invention to provide an electric discharge tube or discharge lamp, which yields high emission-current densities over a long period of time in a reproducible manner.
In accordance with the invention, this object is achieved by an electric discharge tube or discharge lamp having at least one scandate-dispenser cathode, which is composed of a cathode body and a coating having an emissive surface, said cathode body comprising a matrix of at least one refractory metal and/or a refractory alloy and a barium compound which is in contact with the matrix material to supply barium to the emissive surface by means of a chemical reaction with said matrix material, and the coating containing one or more multilayers which may include a bottom layer of tungsten and/or a tungsten alloy, an intermediate layer of rhenium and/or a rhenium alloy and a top layer of scandium oxide, a mixture of scandium oxide and rare-earth metal oxides, a scandate and/or a scandium alloy.
Such a discharge tube or discharge lamp has a long service life because it has a good resistance to ion bombardment in doses up to several 1019 ions/mm2. Such a discharge tube or discharge lamp can for example be used as a high-resolution computer monitor (CMT), in high-resolution television receivers having a display screen aspect ratio of 16:9 and as high-performance X-ray tubes, because it attains a saturation emission-current density i0xe2x89xa625 A/cm2 at 965xc2x0 C., which is the measured radiation temperature of the molybdenum cap of the cathode holder.
Another aspect of the invention relates to a scandate dispenser cathode which is composed of a cathode body and a coating having an emissive surface, said cathode body comprising a matrix of at least one refractory metal and/or a refractory alloy and a barium compound which is in contact with the matrix material to supply barium to the emissive surface by means of a chemical reaction with the matrix material, and the coating comprises one or more multilayers which may include a bottom layer of tungsten and/or a tungsten alloy, an intermediate layer of rhenium and/or a rhenium alloy and a top layer of scandium oxide, a mixture of scandium oxide and rare-earth metal oxides, a scandate and/or a scandium alloy.
The scandate dispenser cathode in accordance with the invention exhibits low tungsten losses and the scandium dispensation in the emissive surface is not passivated during operation of the cathode. The layer structure precludes the diffusion of oxygen towards tungsten.
A scandate dispenser cathode in accordance with the invention, in which the cathode body comprises a scandium compound or a scandium alloy to dispense scandium to the emissive surface, has a very long service life.
Preferably, the multilayer is made of ultrafine particles. Scandate dispenser cathodes having a coating of ultrafine particles have surface structures and surface modulations of particles whose diameter ranges from 1 to 100 nm and hence relatively small radii of curvature in a dense particle and microtip distribution on the macroscopic surface.
Preferably, the multilayer in the coating of the scandate dispenser cathode in accordance with the invention is manufactured by means of laser-ablation deposition. Unlike well-known wet-chemical processes, laser-ablation deposition involves short reaction times. In addition, unlike well-known evaporation methods, the grain-size distribution of the ultrafine particles can be easily controlled.
It is further preferred that the bottom layer, intermediate layer and top layer each have a thickness in the range from 5 to 150 nm. Scandate dispenser cathodes having such layers exhibit excellent emissive properties.
It is particularly preferred that the coating of the scandate dispenser cathodes in accordance with the invention has a thickness in the range from 50 to 1,000 nm, preferably 400 to 600 nm. In this manner, a cathode having a service life of 10,000 h is obtained.